Reversible light sensitive glass

ABSTRACT

A reversible light sensitive glass comprising 100 parts of a base glass, 0.15 to 1.0 parts of Ag and an amount of halogen corresponding to or more than to the chemical equivalence of Ag, in which the base glass consists of, by weight %, SiO 2  48 to 62, Al 2  O 3  7 to 15, B 2  O 3  17 to 26, R 2  O (where R represents Na, Li or K) 6 to 16, BaO 0.5 to 5, (BaO/R 2  O 0.05 to 0.40), TiO 2  0 to 10, ZrO 2  0 to 10, Nb 2  O 5  0 to 5, TiO 2  +ZrO 2  +Nb 2  O 5  0.5 to 15, AS 2  O 3  0 to 2.0, and Sb 2  O 3  0 to 2.0 (AS 2  O 3  +Sb 2  O 3  &lt;2.0).

This application is a continuation-in-part of application Ser. No.232,667 filed Mar. 8, 1972 now U.S. Pat. No. 3,833,511.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a reversible light sensitive glass which has ahigh refractive index (n_(d) value higher than 1.50) and in which thepresent transmission is rapidly and reversibly changed depending on theradiation of ultraviolet rays and visible light of a short wave length.

2. Description of the Prior Art

Generally, a light shielding glass lens must be made from a reversiblelight sensitive glass having a property of rapidly responding to light.If the refractive index is increased, not only can the thickness of theglass lens be reduced but also the usual glass lens grinding dish can beemployed with the glass lens.

The glass lens must have an n_(d) of 1.523 for applying it to the usualglass lens grinding dish, but the modification of the n_(d) value tosuch value is very difficult without reducing the ability of the glassto respond to light (hereinafter, light responding property) and thefading rate.

We, the inventors, have already proposed a reversible light sensitiveglass having a high light responding property and the rapid fading rate,in U.S. Pat. application Ser. No. 232,667, filed on Mar. 8, 1972 whichis based on the discovery that the coexistence of BaO and R₂ O increasesthe light responding property and the rapid fading rate of the glass. Ifthe amount of BaO for obtaining the maximum light responding propertyand fading rate is restricted, the increase of the value of therefractive index to 1.50 or more is impossible.

On the other hand, the commercially available reversible light sensitiveglass contains a large amount of BaO to increase the refractive indexvalue at 1.523 and is used in combination with PbO for the same purposeinspite of the adverse effects of PbO on the light responding propertyand fading rate.

Accordingly, the resultant glass exhibits a poor light respondingproperty and especially a low fading rate. For example, in thecommercial glass the restoration of the up to 90% of the initialtransmission requires about 60 min.

Besides, most reversible light sensitive glasses are not completelydurable to weathering, although weather resistance is requiredespecially for a glass which is always exposed to the atmosphere.

SUMMARY OF THE INVENTION

An object of this invention is to provide a reversible light sensitiveglass having a relatively high refractive index without reducing thelight responding property and fading rate.

Another object of this invention is to provide a reversible lightsensitive glass having high weather resistance.

This invention has been accomplished on the discovery that TiO₂, ZrO₂,Nb₂ O₅ and Ta₂ O₅ in the glass increases the refractive index withoutreducing the fading rate, and also that the ZrO₂ remarkably improves thedurability of the glass, especially the acid resistance.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

The FIGURE shows the influence of TiO₂, ZrO₂, Nb₂ O₅ and Ta₂ O₅, each ofwhich is substituted for a part of the SiO₂ in a glass composed byweight of SiO₂ 60.2%, B₂ O₃ 18.6%, Al₂ O₃ 8.3%, K₂ O 10.9%, Li₂ O 0.9%,BaO 1.2%, Ag 0.3%, Cl 1.1% and CuO 0.005%, on the increase of therefractive index of the glass.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the FIGURE, TiO₂ is most effective to increase therefractive index of the glass, when substituted for a part of the SiO₂.Although TiO₂ is effective to increase the refractive index of theglass, the use of TiO₂ has been avoided conventionally for the followingreasons.

TiO₂ colors greatly such a type of glass, and the colored glass becomesuseless as a glass lens. The Ti ion derived from the TiO₂ absorbsultraviolet rays thereby decreasing the light sensitivity.

Accordingly, one feature of this invention is to introduce As₂ O₃ andSb₂ O₃ into the TiO₂ containing glass thereby to eliminate the defectsof the glass caused by the addition of TiO₂.

It was found that the remarkable coloring of the glass due to the TiO₂addition was unexpectedly extinguished by introducing a small amount ofAs₂ O₃ to the glass. A glass which was a strong red-brown color by theaddition of only 3 weight % of TiO₂ was restored in its transparency tothe equivalent of that without TiO₂ by introducing only 0.02% of As₂ O₃and/or Sb₂ O₃.

Another feature of this invention is to substitute a part of the Cl withBr and I. It was discovered that the reduction in the light sensitivityof the glass due to the absorption of ultraviolet rays by the Ti ioncould be prevented by substituting a part of the Cl with Br and I whichshifted the light sensitive wave length range of the glass towards thelong wave length side.

As described above, this invention makes the use of TiO₂, which is veryeffective to increase the refractive index of the glass, possiblewithout reducing the light sensitivity and the fading rate.

The reversible light sensitive glasses according to this invention are;

1. A reversible light sensitive glass comprising 100 weight parts of abase glass, 0.15 to 1.0 weight parts of Ag and a small amount of ahalogen corresponding to the chemical equivalence of Ag, in which thebase glass is composed of, by weight % SiO₂ 48 to 62, Al₂ O₃ 7 to 15, B₂O₃ 17 to 26, R₂ O (where R represents Na, Li or K) 6 to 16, BaO 0.5 to5, (BaO/R₂ O 0.05 to 0.40), TiO₂ 0 to 10, ZrO₂ 0 to 10, Nb₂ O₅ 0 to 5,TiO₂ +ZrO₂ +Nb₂ O₅ 0.5 to 15, As₂ O₃ 0 to 2.0 and Sb₂ O₃ 0 to 2.0 (As₂O₃ +Sb₂ O₃ <2.0).

2. A reversible light sensitive glass as above described in item (1),wherein the amount of As₂ O₃ +Sb₂ O₃ is 0.02 to 2.0% when the TiO₂amount exceeds 1%, and a part of Cl is substituted with more than 0.1parts of Br+I.

The slightest amount of BaO remarkably increases the color concentrationof the glass, and a coexistence of BaO and R₂ O, especially K₂ O,increases the fading rate. In order to obtain a sufficiently high colorconcentration and fading rate, the addition of more than 0.5% of BaO isrequired to keep the ratio of BaO/R₂ O at a value higher than 0.05.

On the other hand, more than 5.0% of BaO in the glass and a BaO/R₂ Oratio of more than 0.40 cause rather a reduction in the colorconcentration and a low fading rate.

Accordingly, the BaO amount and the BaO/R₂ O ratio advantageously are0.5 to 5.0% and 0.05 to 0.40, respectively.

Less than 6% of R₂ O is not sufficient to exhibit the cooperation of BaOand R₂ O, whereas more than 16% R₂ O decreases the durability of theglass. Accordingly, the advantageous amount of R₂ O is 6 to 16%. Lessthan 7% of Al₂ O₃ and less than 17% of B₂ O₃ does not provide asufficient amount of Al₂ O₃ and B₂ O₃ to exhibit their action togetherwith BaO and R₂ O. Use of more than 15% of Al₂ O₃ does not provide moreeffective results and decreases the fusability of the glass. More than26% of B₂ O₃ makes the glass unstable and decreases the transparency ofthe glass due to phase separation during heat treatment.

Accordingly, the advantageous amount of Al₂ O₃ and B₂ O₃ are 7 to 15%and 17 to 26%, respectively.

A glass having a high color concentration and a rapid fading rate cannotbe formed with more than 62% of SiO₂, and the fusing of the glass isdifficult with an SiO₂ level of above 62%. On the other hand, theaddition of less than 48% of SiO₂ decreases the chemical durability ofthe glass and increases phase separation during the heat treatment.Accordingly, the advantageous SiO₂ amount is 48 to 60%.

More than 10% of TiO₂ and ZrO₂ makes the glass unstable and reduces thefading rate.

Since Nb₂ O₅ is very expensive the addition of more than 5% of Nb₂ O₅ isnot suitable as raw material of the glass lens.

Ta₂ O₅ is effective to improve the light responding properties of theglass but is also very expensive. If the sum of TiO₂, ZrO₂ and Nb₂ O₅ isdecreased below 0.5%, a glass having a refractive index greater than 1.5can not be produced. On the other hand, more than 15% of TiO₂ +ZrO₂ +Nb₂O₅ causes the glass to be unstable and decreases the fading rate. Morethan 2% of As₂ O₃ and/or Sb₂ O₃ makes the precipitation of crystalgrains of Ag halide in the glass difficult, thus greatly decreasing thelight sensitivity.

Especially, more than 0.02% of As₂ O₃ +Sb₂ O₃ remarkably prevents thecoloring of the glass due to the TiO₂ present therein at a content ofmore than 1%.

The light sensitive components added to 100 parts of such a base glasscomposition are Ag 0.15 to 1.0 parts and halogen in an amountcorresponding to or more than the chemical equivalence of Ag.

An addition of less than 0.15 parts of Ag is not sufficient to form thefull amount of Ag halide in the glass and to obtain the full colorconcentration. On the other hand, more than 1.0 parts of Ag causes theglass to be a light milky opaque, and the resultant glass is not usefulfor a glass lens.

Sufficient color concentration can not be obtained with a halogen amountbelow the chemical equivalence of Ag.

When more than 1% of TiO₂ exists in the glass, more than 0.1 parts ofBr+I shifts the light sensitive wave length range towards the long wavelength side preventing a reduction in the light sensitivity.

A very small amount of CuO increases the color concentration, but morethan 0.015% of CuO is rather ineffective.

Some Examples of this invention are shown by reference to the followingTable.

    __________________________________________________________________________    1          2     3     4     5     6     7     8     9                        __________________________________________________________________________    SiO.sub.2                                                                          55.2  52.2  59.2  50.0  53.9  54.2  51.2  50.6  50.6                     B.sub.2 O.sub.3                                                                    18.6  21.6  13.6  16.6  18.6  18.9  18.6  18.6  17.6                     Al.sub.2 O.sub.3                                                                   8.3   8.3   8.3   14.0  8.3   8.5   11.5  5.3   5.3                      K.sub.2 O                                                                          10.9  10.9  10.9  9.9   10.9  8.2   8.2   10.9  8.9                      Na.sub.2 O                                                                         --    --    --    --    --    --    0.7   2.0   0.6                      Li.sub.2 O                                                                         0.9   0.9   0.9   0.9   0.9   1.8   1.8   0.9   0.9                      BaO  1.2   1.2   1.2   1.2   1.0   1.0   1.0   1.2   1.2                      TiO.sub.2                                                                          --    --    5.0   2.5   6.3   5.3   4.9   3.0   6.0                      ZrO.sub.2                                                                          5.0   --    --    4.0   --    1.9   1.9   6.0   6.0                      Nb.sub.2 O.sub.5                                                                   --    5.0   --    --    --    --    --    1.6   3.0                      As.sub.2 O.sub.3                                                                   --    --    0.5   0.9   0.2   0.1   --    0.03  --                       Sb.sub.2 O.sub.3                                                                   --    --    0.4   --    --    0.1   0.2   --    0.05                     Ag   0.3   0.3   0.3   0.3   0.3   0.6   0.6   0.3   0.6                      Cl   1.1   1.1   1.1   1.1   1.1   0.6   0.6   0.3   0.6                      Br   --    --    --    --    --    0.5   --    0.8   0.6                      I    --    --    --    --    --    --    0.5   --    --                       CuO  0.005 0.005 0.005 0.005 0.005 0.005 0.08  0.005 0.007                    n.sub.d                                                                            1.502 1.506 1.516 1.513 1.523 1.525 1.524 1.523 1.531                    *A.R(%)                                                                            0.12  0.17  0.23  0.20  0.18  0.19  0.17  0.25  0.25                     **T.sub.90                                                                     (min)                                                                             10    5     15    15    16    15    15    20    20                       __________________________________________________________________________     *A.R (%)- Acid resistance as shown by weight loss (%)                         **T.sub.90 (min)- Time required for restoring 90% of the initial              transmission                                                             

The fading velocity of the glass of this invention is indicated by timeperiod (T₉₀) for restoring 90% of the initial transmission of the glassafter the light irradiation was ceased. The coloring of the glass wascarried out by irradiation with light from a 500W Xenon lamp on a glassplate of a thickness of 3 mm at a distance of 10 cm from the lightsource and the fading was done in the absence of irradiation. Thepercent transmission of the glass at each time was measured using aspectrophotometer at a wavelength 550 mμ.

The glass of this invention can be produced by melting a mixture of eachof the components in a platinum or ceramic crucible at 1400° to 1500° C,(stirring) the melt and pouring the melt into a mold, and pressingtherein. The glass obtained is heat treated at a temperature rangingbetween the transformation temperature and the softening temperature forseveral minutes to several hours whereby a reversible light sensitiveglass having a rapid coloring rate and fading rate is obtained.

As shown in the Examples, the glasses of this invention exhibit an n_(d)value of more than 1.50 in spite of their high coloring and fadingrates. A commercially available reversible light sensitive glassrequired about 60 minutes for a restoration of 90% of its transmission,whereas the glass of this invention required only 5 to 20 minutes forthis purpose.

Since the n_(d) value of the glass of this invention can be easilyadjusted to 1.523, a specially shaped grinding tray is not required.According to this invention, glasses having a higher n_(d) value can beproduced, and thinner glass lens can be produced.

While this invention has been described in detail and with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made therein by those skilled in the art withoutdeparting from the scope of this invention.

Therefore, the appended claims are intended to cover all such equivalentvariation as coming within the true spirit and scope of this invention.

What is claimed is:
 1. A reversible light sensitive glass comprising 100weight parts of a base glass, 0.15 to 1.0 weight parts of Ag and anamount of halogen corresponding to or more than the chemical equivalenceof the Ag, in which the base glass consists, in weight %, of SiO₂ 48 to62, Al₂ O₃ 7 to 15, B₂ O₃ 17 to 26, R₂ O, where R represents Na, Li orK, 6 to 16, BaO 0.5 to 5, BaO/R₂ O 0.05 to 0.40, TiO₂ 0 to 10, ZrO₂ 0 to10, Nb₂ O₅ 0 to 5, TiO₂ +ZrO₂ +Nb₂ O₅ 0.5 to 15, As₂ O₃ 0 to 2.0, Sb₂ O₃0 to 2.0 and As₂ O₃ +Sb₂ O₃ <2.0.